1. Field of the Invention
The invention relates generally to steelmaking, particularly to the production of steel in side-tapped vessels.
2. Description of the Prior Art
The production of steel in substantially cylindrical vessels has been practiced since the inception of the Bessemer process in the mid-1800's. The steelmaking process has remained essentially unchanged in its major steps to date, although advances such as the basic oxygen furnace have been made.
One of the key steps in steelmaking is the formation of a layer of molten slag, which is necessary to the chemical conversion of the molten pig iron to steel, particularly with respect to the removal of impurities from the charged materials. The slag comprises a molten liquid having a lower density than the steel, such that it floats on the molten steel as an essentially unitary layer. On the completion of the heat, it is necessary to separate the slag layer from the steel. The less slag which is left on the liquid steel, the higher the overall purity of the steel products (bar, billet and the like) which will result on casting.
In substantially cylindrical steelmaking vessels, two alternatives were known for removing the molten materials, slag and steel, at the completion of the heat. The simplest was to turn or tilt the vessel, which is usually mounted on trunions to facilitate charging and discharging of materials, so as to pour the molten materials over the lip. The slag/steel density difference allows the pouring-off of the slag first, followed by the steel, if the steelmaker can control the progressive tilt of the vessel with sufficient accuracy to provide a slow, steady stream of molten material.
This technique presented numerous difficulties. The appropriate tilt to result in substantially complete slag removal before discharge of steel was almost impossible to control, particularly with respect to consistently attaining the same flow and separation. Too much tilt resulted in too rapid a flow, producing turbulence and the intermingling of the slag and steel, with the result of wasted steel removed with the slag and impure steel remaining in the vessel. Furthermore, pouring over the lip of the vessel was deleterious to the vessel and its internal refractory lining. The stream of molten material was also difficult to control, as slag and steel would build up on the vessel lip.
The second alternative, which is the most followed in the steelmaking art in tapping a substantially cylindrical steelmaking vessel, utilized a taphole in the side wall of the vessel. The taphole is closed during the course of the heat.
On completion of the heat, the vessel was tilted on its trunions until the slag-steel layer boundary contacted the side wall on which the taphole was mounted at a point substantially above the upper edge of the taphole. The taphole was then opened and the steel removed from below the slag layer, which continued to float on the molten steel. The steel was removed by progressively tilting the vessel until slag started to flow out of the taphole, at which time the tapping was stopped.
This second alternative also resulted in difficulties. The degree and progression of tilt was once again difficult to control. Too rapid a flow again resulted in turbulence, the intermingling of the slag and steel, and the attendant waste and impurity problems. The deliberate continuation of tapping until slag began to be discharged from the vessel presented the problem of having to remove that slag by skimming or other methods from every heat tapped.
To attempt to alleviate the difficulties with the taphole procedure, the steelmaking art developed the improvement to the tapping procedure of adding at least one discrete, usually spherically-shaped piece of a heat-stable material having a density such that it floated substantially at the slag-steel interface, to the vessel prior to the onset of tapping. When the majority of the molten steel was removed from the vessel on tapping, a sphere of the heat-stable material would flow towards and block off the taphole, allowing only a slight amount of slag into the tapped steel.
Even with this improvement, however, a serious difficulty remained. The taphole in the substantially cylindrical steelmaking vessels was round. When liquid is drained through a round hole, a vortex is generated which often extends well into the liquid. Control of the formation of a vortex was found to be extremely difficult in tapping steelmaking vessels.
Minimal vortexing could be assured only by a very slow tapping rate, which resulted in poor heat cycle time and overcooling of the steel during tapping. When the heat-stable material was used, even minimal vortexing posed the threat of drawing in the discrete piece and closing off the taphole prematurely, leaving too much untapped steel in the vessel, particularly as the vortexing increases as the overall depth of the liquid layer decreases. Attainment of a satisfactory cycle time thus required the tapping to take place at a rate which resulted in more than minimal vortexing and the leaving in the vessel of essentially all of the slag together with a substantial quantity of steel.
Despite the improvements to controlling slag carry-over during tapping, then, the problem of vortexing remained, and in fact helped turn the improvement of use of at least one discrete piece of heat-stable material into the source of yet another problem to the steelmaker.